On Wed, Aug 18, 2021 at 8:23 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > On 8/17/2021 4:06 PM, Kees Cook wrote: > > On Tue, Aug 17, 2021 at 04:33:25PM -0500, Gustavo A. R. Silva wrote: > >> > >> > >> On 8/17/21 16:17, Masahiro Yamada wrote: > >>> On Wed, Aug 18, 2021 at 3:25 AM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > >>>> > >>>> On 8/17/2021 11:03 AM, Kees Cook wrote: > >>>>> On Mon, Aug 16, 2021 at 09:55:28PM -0700, Nathan Chancellor wrote: > >>>>>> If you/Gustavo would prefer, I can upgrade that check to > >>>>>> > >>>>>> ifneq ($(call cc-option, -Wunreachable-code-fallthrough),) > >>>>>> > >>>>>> I was just trying to save a call to the compiler, as that is more expensive > >>>>>> than a shell test call. > >>>>> > >>>>> I prefer the option test -- this means no changes are needed on the > >>>>> kernel build side if it ever finds itself backported to earlier versions > >>>>> (and it handles the current case of "14" not meaning "absolute latest"). > >>>>> > >>>>> More specifically, I think you want this (untested): > >>>> > >>>> That should work but since -Wunreachable-code-fallthrough is off by > >>>> default, I did not really see a reason to include it in KBUILD_CFLAGS. I > >>>> do not have a strong opinion though, your version is smaller than mine > >>>> is so we can just go with that. I'll defer to Gustavo on it since he has > >>>> put in all of the work cleaning up the warnings. > >>> > >>> > >>> > >>> https://github.com/llvm/llvm-project/commit/9ed4a94d6451046a51ef393cd62f00710820a7e8 > >>> > >>> did two things: > >>> > >>> (1) Change the -Wimplicit-fallthrough behavior so that it fits > >>> to our use in the kernel > >>> > >>> (2) Add a new option -Wunreachable-code-fallthrough > >>> that works like the previous -Wimplicit-fallthrough of > >>> Clang <= 13.0.0 > >>> > >>> > >>> They are separate things. > >>> > >>> Checking the presence of -Wunreachable-code-fallthrough > >>> does not make sense since we are only interested in (1) here. > >>> > >>> So, checking the Clang version is sensible and matches > >>> the explanation in the comment block. > > > > I thought one of the problems (which is quickly draining away) that > > needed to be solved here is that some Clang trunk builds (that report > > as version 14) don't yet have support for -Wunreachable-code-fallthrough > > since they aren't new enough. > > Philip, how often is the kernel test robot's clang version rebuilt? > Would it be possible to bump it to latest ToT or at least > 9ed4a94d6451046a51ef393cd62f00710820a7e8 so that we do not get bit by > this warning when we go to enable this flag? > > I do not know of any other CI aside from ours that is testing with tip > of tree clang and ours should already have a clang that includes my > patch since it comes from apt.llvm.org. > > >>> # Warn about unmarked fall-throughs in switch statement. > >>> # Clang prior to 14.0.0 warned on unreachable fallthroughs with > >>> # -Wimplicit-fallthrough, which is unacceptable due to IS_ENABLED(). > >>> # https://bugs.llvm.org/show_bug.cgi?id=51094 > >>> ifeq ($(firstword $(sort $(CONFIG_CLANG_VERSION) 140000)),140000) > >>> KBUILD_CFLAGS += -Wimplicit-fallthrough > >>> endif > > Very clever and nifty trick! I have verified that it works for clang 13 > and 14 along with a theoretical clang 15. Gustavo, feel free to stick a I am not the inventor of this code, though :-) I mimicked the code in Buildroot: https://github.com/buildroot/buildroot/blob/2021.05/Makefile#L104 -- Best Regards Masahiro Yamada
